1. Field of the Invention
The present invention relates to a method of producing an aldehyde compound in which a carboxylic acid or its ester is reduced to prepare a corresponding aldehyde compound.
2. Description of the Related Art
Aldehyde compounds play an important role in the field of organic chemical industries. For example, they are widely used as the raw material for perfumes or as intermediates for medicines, agricultural chemicals, and the like. They are also used for producing a higher alcohol, through utilizing aldol condensation, or for producing alcohols by way of reduction. Since the aldehydo group is active, aldehyde compounds are also used for synthesizing fine chemicals.
Methods for industrially synthesizing an aldehyde compound includes the oxo process, which makes use of an olefin as a raw material, and causes carbon monoxide and hydrogen to react therewith, to synthesize an aldehyde. However, this method is normally used only for synthesizing a lower aldehyde, since it is difficult to synthesize a higher aldehyde or a branched aldehyde by using this method (see "INDUSTRIELLE ORGANISCHE CHEMIE" by K. Baiserumar, H. J. Alpe).
A method, generally called "partial oxidation", is often used to oxidize alcohol, thereby to synthesize an aldehyde compound on an industrial scale. This method uses, as an oxidizing agent, chromic acid, pyridinium chromate, pyridinium chlorochromate, manganese dioxide, or a combination of dimethyl sulfoxide and electrophilic reagent. However, when this method is used, the alcohol is all too readily oxidized to a carboxylic acid. It is difficult to stop the oxidation of the alcohol before the produced aldehyde is further oxidized to a carboxylic acid. In order to stop the oxidation at this time, reaction conditions, such as the temperature, the solvent, and the like, must be strictly controlled. Moreover, since this method entails the use of an oxidizing agent which contains a heavy metal such as chromium, manganese, or the like, this results, undesirably, in a large amount of harmful waste being produced as a by-product of the reaction.
Another method of industrially synthesizing an aldehyde compound has been reported, which reduces a carboxylic acid to an aldehyde ("Chemistry Letters", 1974, 1447). In this method, aminoalane prepared from a secondary amine such as N-methyl piperazine and an aluminum hydride is used as a reducing agent. However, the reducing agent is readily hydrolyzed by water and becomes inactive. In addition, the reducing agent is expensive, and must be used in an amount more than an equivalent amount, resulting in high producing cost. Furthermore, since the partial reduction method using aminoalane is a homogenous reaction as in the partial oxidation method of the alcohol, it is not easy to separate a produced aldehyde, and a countermeasure for preventing corrosion of a reaction apparatus is also necessary.
As other methods of reducing a carboxylic acid to synthesize an aldehyde, a method of performing reduction using Na-Hg (Ber. duet. chme. Ges. 41, 4147 (1908)), and a method of reducing a carboxylic acid by formic acid using manganese monoxide, titanium dioxide, or the like as a catalyst (Compt. rend. 154, 561 (1912), J.C.S.70, 86 (1943)), have been reported. However, the latter method has very low reduction activity except for salicylic acid.
Note that a method of reducing a carboxylic acid ester to produce an aldehyde is unknown.